AJP - Endocrinology and Metabolism, Vol 252, Issue 1 E38-E43, Copyright © 1987 by American Physiological Society
Regulation of 25-hydroxyvitamin D3 metabolism in chick embryo
R. T. Turner, J. S. Graves and N. H. Bell
We investigated the time course of the development of renal
25-hydroxyvitamin D-1-hydroxylase and 25-hydroxyvitamin D-24-hydroxylase in
chick embryos grown in the presence and absence of the eggshell. In embryos
with the eggshell, the specific activity (SA) of 25-hydroxyvitamin
D-1-hydroxylase in kidney homogenates increased from 0.68 fmol X min-1 X mg
protein-1 at 12 days of gestation to a peak of 2.55 +/- 0.50 fmol X min-1 X
mg-1 protein-1 at 17 days. In contrast, the SA of 25-hydroxyvitamin
D-24-hydroxylase decreased from 2.5 fmol X min-1 X mg protein-1 to 0.90 +/-
0.25 fmol X min-1 X mg protein-1 during the interval. The total plasma
calcium was significantly reduced in embryos without shells at 14 to 15
days of gestation (1.1 +/- 0.1 mM, mean +/- SE) compared with normal
embryos of the same gestation (2.3 +/- 0.3 mM, P less than 0.002). In
embryos without the eggshell, renal 25-hydroxyvitamin D-1-hydroxylase
increased from 6.0 to 8.2 +/- 0.6 fmol X min-1 X mg protein-1 at 17 days of
gestation and was from four- to sixfold higher than corresponding enzymatic
activities for intact embryos. The increased enzyme activity resulting from
loss of the eggshell was due to an increase in Vmax. The findings indicate
that renal 25-hydroxyvitamin D-1-hydroxylase and 25-hydroxyvitamin
D-24-hydroxylase in the chick embryo exhibit activity and show a large
capacity for regulation in response to changes in calcium metabolism.
